1. Field of the Invention
The present invention relates to methods and apparatus for controlling the force applied to a drill bit while drilling and, more particularly, to such methods and apparatus which vary the flow of fluid circulating in a string of drill pipe to achieve such control.
2. Setting of the Invention
A common configuration for drilling wells includes a drill bit suspended from a string of drill pipe. Often, the drill bit includes openings in the lower end thereof to enable circulation of drilling fluid down the pipe string, through the bottom of the bit, and upwardly into the annulus between the outer surface of the pipe string and the wellbore. In conventional rotary drilling, the drill string is rotated while the fluid circulation flushes cuttings from the bottom of the wellbore and cools the drill bit.
In another form of drilling, a cylindrically-shaped downhole hydraulic motor is suspended from the lower end of a string of drill pipe and a drill bit having openings in the lower end thereof is mounted on a tubular drive shaft which extends from the lower end of the motor. Fluid is circulated down the drill string and through the motor thereby rotating the drive shaft. Fluid continues through the drive shaft, out of the bottom of the bit, and into the annulus. Thus, the fluid powers the downhole motor and, as in conventional rotary drilling, flushes cuttings from the bottom of the hole and cools the drill bit during drilling.
The destructive energy which is generated between the drill bit and the rock is related directly to the weight which is applied to the drill bit. In normal drilling operations of both the rotary and downhole-motor type, the weight applied to the bit is controlled at the surface of the well by varying the force used to suspend the drill string. For some types of bits (i.e., drag bits, diamond bits), the effective weight applied to the bit is also affected by the flow rate of circulating fluid through the bit. As the fluid flows from the drill string, through the bit, and into the wellbore beneath the bit, an upward force is generated by the pressure differential below and above the bit. This force is equal to the pressure drop times the effective pressurized area of the bit and tends to reduce the weight applied to the bit as it increases. This force is commonly referred to as the "pump-off effect."
The bit weight required to maintain a particular energy level varies with changes in rock properties, drill bit dullness, and the quality and quantity of fluid circulating through the bit. Such an energy level can be maintained by applying a substantially constant amount of torque to the drill bit which maximizes the rate of penetration while avoiding rapidly dulling or destroying the drill bit.
Several problems arise when attempting to apply a constant level of torque to the drill bit. Both the weight applied to the bit and rotary torque applied to the bit as measured at the surface are inaccurate due to wellbore friction acting on the drill string. Although there exists commercially available devices for measuring weight applied to the bit and drill bit torque at the bit, when such are used to transmit information to the surface to vary the force used to suspend the drill string, the response time is insufficient to avert drill bit failure when the property of rock through which the bit is drilling suddenly changes.
In the case of drilling with downhole motors, additional problems are encountered. When using a turbine-type downhole motor, maximum turbine output power is achieved at a selected output torque and rate of rotation. Since bit torque increases nearly linearly as the weight applied to the drill bit increases, it is desirable to operate with a weight applied to the drill bit which permits the turbine to operate at its maximum output power. Since, as noted above, it is difficult to determine the weight applied to the bit and to maintain this weight during drilling, turbines rarely operate at their peak power. When the weight applied to the bit is less than that required to obtain peak power, the drill bit is underloaded and underutilized. When the weight applied to the drill bit is greater than that which produces peak power, the drill bit is excessively worn.
A positive-displacement-type downhole motor operates at its peak power very near its stall point. Thus, variations in the weight applied to the drill bit, which vary the amount of torque necessary to turn the bit, can stall the motor.
There exists a need for a method and apparatus for controlling the advancement of, as well as the load applied to a drill bit. Moreover, there exists a need for such a method and apparatus in which the weight-on-bit is controlled to obtain a constant drill bit torque.